(1) Field of the invention
The present invention generally relates to a printing head in a wire-dot printer, and particularly to a printing head in a wire-dot printer in which the head has two types of wire pins.
(2) Description of related art
In a wire-dot printer, a recording sheet is put between a printing head and a platen, and wire pins of the printing head hammer the recording sheet via an ink ribbon against the platen, so that characters corresponding to the arrangement of the hammering wire pins are printed on the recording sheet. Recently, high speed wire-dot printers have become desired with the increasing amount of printing needed in offices, factories and distribution sections. To improve the printing rate (the printing speed), increasing the number of wire pins provided in a wire-dot printer is a valid approach.
A conventional wire-dot printer is shown in FIG. 1. Referring to FIG. 1, a wire-dot printer has a paper feed unit 1, a printing head unit 2 and a shuttle unit 3. While the printing head unit 2 having a large number of wire pins is being moved from side to side by the shuttle unit 3, printing of characters is performed.
The paper feed unit 1 has a platen 4 and a tractor 5 for feeding a recording paper 100 (a continuous form) upward. The platen and the tractor 5 are driven by a feeding motor 6. The printing head unit 2 has a plurality of heads 7 arranged side to side. Each of heads 7 has a plurality of wire pins which is arranged in a line as shown in FIG. 2A. The wire pins provided in each of the heads 7 may be arranged in a slanting line, as shown in FIG. 2B. The shuttle unit 3 has a linear motor, and moves the printing head 2 from side to side within a predetermined region. While the printing head unit 2 is being moved by the shuttle unit 3, wire pins selected in accordance with image data hammer the recording paper 100 via an ink ribbon (not shown), supplied by an ink ribbon supplier (not shown), against the platen. Due to the hammering of the wire pins, characters corresponding to the image data are successively printed on the recording paper 100.
There are various types of printing head, such as a disengaging type, a moving-coil type and a piezo-electric type.
FIG. 3A shows the disengaging type printing head 7A. Referring to FIG. 3A, two head modules 7A.sub.1 and 7A.sub.2 are fixed on a frame 11 so that the head modules 7A.sub.1 and 7A.sub.2 face each other in a vertical direction. Each of the head modules 7A.sub.1 and 7B.sub.2 has a wire pin unit 12 formed of a leaf spring 8, an armature 9, a beam 10 and a wire pin 10a and a magnet unit 16 formed of a magnet 13, a coil 14 and a core block 15. In the wire pin unit 12, the leaf spring 8 and the armature 9 are successively connected to each other by a welding process. A wire pin 10a is mounted on an end of the beam 10 so as to be inserted into a pin guide 17. The armature 9 is normally attracted by the core block 15, and the armature 9 is disengaged from the core block 15 by the righting moment of the leaf spring 8 when an electric current is supplied to the coil 14. Due to the disengagement of the armature 9 from the core block 15, the wire pin 10a projects from the pin guide 17, and the tip end of the wire pin 17 then hammers the recording paper via the ink ribbon. In a 24-dot-matrix printing type head, the printing end of the wire pins of the printing head are arranged, for example, in an array of straight lines as shown in FIG. 3B.
FIG. 4A shows a moving coil type printing head 7B, and FIG. 4B shows an essential part of the moving coil type printing head. Referring to FIGS. 4A and 4B, two head modules 7B.sub.1 and 7B.sub.2 are provided on a supporting block so as to face each other in a vertical direction. In each of the head modules 7B.sub.1 and 7B.sub.2, a flat coil 18 is mounted on a surface of a base 19, and a set of the flat coil 18 and the base 19 is arranged in each interval between the magnets 20. When an electric current is supplied to the flat coil 18, the flat coil 18 is moved in a direction indicated by an arrow in FIG. 4B, so that a wire pin 21 mounted on an end of the base 19 by welding process projects from the supporting block.
FIG. 5A shows a piezo-electric type printing head 7C, and FIG. 5B shows an essential part of the piezo-electric type printing head. Referring to FIGS. 5A and 5B, two head modules 7C.sub.1 and 7C.sub.2 are mounted on a supporting block 29 so as to face each other in a vertical direction. Each of the head modules 7C.sub.1 and 7C.sub.2 comprises a plurality of printing elements 28. Each of the printing elements 28 shown in FIG. 5B is formed of a base member 22, a piezo-electric element 23 (a laminated type), a block 24, two leaf springs 25.sub.1 and 25.sub.2, a beam 26 and a wire pin 27. When a voltage is supplied to the piezo-electric element 23, the piezo electric element 23 expands slightly. The amount of expansion of the piezo-electric element 23 is enlarged by the leaf springs 25.sub.1 and 25.sub.2, and the beam 26 is moved so that the wire pin 27 mounted on an end of the beam 26 projects from a pin guide 30 formed on the supporting block 29.
In the above various types of wire-dot printer, to increase the number of wire pins, two head modules are mounted on the supporting block so as to be close to each other in the vertical direction. Further, to print kanji characters each formed of a 24.times.24 dot matrix at a high rate, 24 wire pins are arranged in a slanting line, as shown in FIG. 3B. Due to the arrangement of the wire pins, the printing rate was improved, for example, from 150 LPM (lines per minutes) to 300-400 LPM.
However, as the intervals at which the wire pins are arranged depends on the size of the coil, the width of the leaf spring and the like, the number of wire pins capable of being provided in the printing head is limited.